The present invention relates to a condenser apparatus and method for condensing gases in two subsequent stages, in the last of which the gases are condensed by means of an absorption medium.
The existence of noncondensable gases is, as is well known, a problem substantially affecting the design and development of a condenser. If it is not possible to effectively separate them from the condensable gases, it is necessary to pump great amounts of them out of the process, which again results in great losses of energy and material.
In some processes, as in condensing of water vapor the question is only of separating condensable and noncondensable gases, generally steam and air, from each other by cooling the gas mixture. In some cases, as for example, in chemical pulp and paper industry there are, also third type gases which gases are noncondensable at the normally used pressures and temperatures, but said gases may be separated from the gas mixture by absorbing them to an appropriate absorption medium.
When a mixture of three such different types of gases is condensed in a conventional condenser, all the noncondensable gases are pumped from the condenser and, according to the pressure and temperature, also a lot of steam with them.
Another known technique is to condense the gas mixture to an absorption medium. Since condensable gases, such as water vapor, forms the majority of the gas mixture being treated, the absorption medium considerably concentrates from the condensing material. Thus it becomes necessary to concentrate it separately by vaporizing the condensed material therein again. This again results in additional use of energy.
It is possible to avoid or minimize the problems of the known methods by using the method in accordance with the present invention. It is not necessary to re-evaporate the condensable material nor is it necessary to pump huge amounts of steam with the noncondensable gases.
The present invention is characterized by a condenser, which comprises cooled heat exchangers of some known construction and which is divided relative to the flow into two subsequent sections, in the latter of which a medium for absorbing gases flows on the surfaces of the heat exchangers, and the noncondensable gases are discharged from the condenser after they have flowed through both sections.
Such a condenser may be located in one casing and the first and second section may be separated by a partition, over which the gases may flow from the first section to the second. It may also be installed into two separate casings, which are connected by the flow channel of gases.
In the first section the majority of the condensable gases is condensed and becomes liquid on the surfaces of the heat exchangers. When the gas mixture at the same time cools, the partial pressure of the noncondensable gases increases. A mixture including a substantial amount of non-condensible gases flow to the second section of the condenser, wherein the noncondensable, but yet absorbable gases are absorbed into the absorption medium flowing along the surfaces of the heat exchangers.
The absorption medium may be a concentrated solution of an electrolyte, for example NaOH, in which the vapor pressure of the water therein is considerably lower than that of pure water at a corresponding temperature. Therefore, the water vapor in the gas mixture also absorbs into this kind of absorption medium. In order to avoid the absorption of unnecessarily large mixture amounts to dilute the absorption solution, the first stage of the condenser must be dimensioned in such a way that the gas mixture cools sufficiently and that the condensing of the water vapor therein would be as complete as possible.
This heat exchangers of the second section cool the absorption medium so as not to permit the vapor pressure increase when the gases absorb, but that the pressure of the second section would remain below the pressure of the first section. In order to create a sufficient rinsing of the heat exchangers, it is possible to return absorption liquid with a pump to flow again to the heat exchange surfaces.
When the absorbable gases have absorbed in the second section of the condenser, the partial pressure of the actual noncondensable gases has grown significant and the gases are discharged from the condenser after they have flowed through both sections.
An example of the prior art technique is an evaporation plant of black liquor in a pulp mill. The temperature of the vapor in the last stage is usually about 55.degree. C., although the cooling water would be only 15.degree. C. Such a great difference in the temperature is used for cooling the gas mixture over 20.degree. C., in order to maintain the vapor amount flowing with the noncondensable gases moderate.
The great temperature difference required by the discharge of gases thus determines the temperature of the last stage. If it could be less, the heat exchangers of an evaporation plant could be built much smaller than they are today. Or alternatively, energy could be saved by building more evaporation stages.